These two verses play on the contrast between sorrow and joy. Even though the language of verse 9 indicates the absence of physical sorrow, nevertheless there will be sorrow in this mortality “for the sins of the world.” This sorrow in the world is then contrasted with the “fullness of joy…in the kingdom of my Father.”
The Savior had told the Nephite multitude “Therefore I would that ye should be perfect even as I, or your Father who is in heaven is perfect. (3 Nephi 12:48.)” That command was to continue on the process of perfection until they reached that state (see the commentary following that verse). For the three Nephites, they are promised that the result of this process is that they will achieve this result, and that they will become as the Father and Son are. This is the state of perfection or Godhood.
“The Latter-day Saint view of man’s potential is in the vanguard of religious or scientific thought. Before the gospel was restored, no one was heard to say, “As God is, man may become,” and yet Jesus said, “Be ye therefore perfect, even as your Father which is in heaven is perfect.” (Matt. 5:48.) Does not that injunction imply limitless possibilities? And the Apostle John said, “Beloved, now are we the sons of God, and it does not yet appear what we shall be: but this we know that, when he shall appear, we shall be like him; for we shall see him as he is.” (John 2:2.)” (Hugh B. Brown, Continuing the Quest [Salt Lake City: Deseret Book Co., 1961], 202 - 203.)